Work machines, including wheel loaders, dump trucks, motor graders, excavators, and other types of heavy machinery, are used for a variety of tasks. These work machines may include mechanisms that are hydraulically actuated to affect steering of the work machine in order to accomplish the tasks. For example, a work machine may include an articulated joint and an associated hydraulic cylinder connected to the work machine. The hydraulic cylinder may extend or retract in response to pressurized fluid from a main source such as, for example, a power steering pump. The expansion and retraction of the hydraulic cylinder may cause a forward end of the work machine to pivot about the articulated joint relative to a back end of the work machine, thereby affecting steering of the work machine.
Work machines may be provided with a backup source of pressurized fluid to provide emergency power steering fluid in the event of failure of the power steering pump or during failure of a power source driving the power steering pump. One such example is described in U.S. Pat. No. 5,022,481 (the '481 patent) issued to Carter et al. on Jun. 11, 1991. The '481 patent describes an emergency power steering system backup apparatus for supplying a vehicle's power steering unit with an emergency supply of power steering fluid when the vehicle's primary power steering pump fails. The apparatus includes a manually-charged emergency power steering fluid supply that is activated when the power steering pump pressure falls below a minimum level, while the vehicle is traveling at a speed above a minimum activating speed. The manually-charged emergency power steering fluid supply includes a supply of hydraulic fluid and a container of compressed gas. When the emergency fluid supply is activated, the compressed gas is directed into the fluid container to drive the fluid to the vehicle's power steering unit.
While the emergency power steering backup apparatus of the '481 patent may provide some steering control of the work machine during failure of the power steering pump, the manually-charged emergency power steering fluid supply of the '481 patent may be insufficient if the work machine is traveling at high speeds or on sloped terrain. In particular, because the emergency supply of power steering fluid is finite, the supply may be exhausted before the work machine is brought to a complete stop. Further, because the emergency supply is manually charged, the supply must be manually recharged after each use. Neglecting to recharge the system could result in little or no backup supply during a subsequent power steering pump failure.
The emergency power steering backup apparatus of the '481 patent may also consume design space and be expensive. Specifically, because the backup apparatus of the '481 patent is additive and serves no purpose other than during power steering pump failure, the apparatus may consume space otherwise available for design of primary work machine systems. Similarly, because the backup apparatus of the '481 patent is dedicated to only providing emergency backup steering, the backup apparatus increases the cost of the base work machine.
The disclosed steering system is directed to overcoming one or more of the problems set forth above.